SUMMARY Chronic inflammation is associated with autoimmune diseases, including rheumatoid arthritis, and is responsible for high morbidity and mortality rates in western society. Consequently, developing effective strategies to prevent or inhibit inflammation is a highly significant objective that will profoundly impact human health care. Activation of vascular endothelial cells (EC) plays a crucial role in the pathology of chronic inflammatory diseases. Our long-term research goal is to identify the specific intracellular signals that underlie EC activation, as this will reveal targets for novel therapeutic strategies aimed at preventing or treating chronic inflammation. A major signaling mechanism associated with inflammation is activation of the NF-?B family of transcription factors. Two independent mechanisms of NF-?B signaling named the classical and non-canonical pathways have been described and each regulates discrete functions. For example, extensive studies have established key roles for classical NF-?B signaling in the pro-inflammatory function of EC and we recently defined a major function for EC-intrinsic classical NF-?B in arthritis. However, emerging evidence strongly supports a role for the non-canonical pathway during the development of chronic inflammation. Our published work and preliminary data has demonstrated that ligation of the lymphotoxin-? receptor (LT?R) activates the non-canonical NF-?B pathway in EC and upregulates expression of crucial pro-inflammatory and pro- angiogenic genes. To date however, no genetic models exist to directly study non-canonical NF-?B signaling in EC in vivo and there are no pharmacological approaches available to specifically target the non-canonical pathway. To address these significant roadblocks, we have developed a new in vivo mouse model conditionally lacking non-canonical NF-?B signaling in EC. In addition, we pioneered an innovative pharmacological approach to selectively target pathway-specific inhibitory polypeptides to activated EC in vivo. The goal of this proposal is to determine the precise function of non-canonical NF-?B signaling in EC and to test our overarching hypothesis that ?non-canonical NF-?B signaling regulates the pro-inflammatory function of endothelial cells?. Accordingly, we will pursue the following three specific aims: (1) To determine how endothelial cell-intrinsic non-canonical NF-?B signaling controls angiogenesis; (2) To define the role of endothelial cell-intrinsic non-canonical NF-?B signaling in arthritis; (3) To selectively pharmacologically inhibit non-canonical NF-?B activation in endothelial cells. These studies will for the first time directly address the effects of selectively abrogating non-canonical NF-?B signaling in EC on the pro-inflammatory function of this critical cell type. Accomplishing our aims will provide novel insight into the potential therapeutic value of targeting non-canonical NF-?B signaling in arthritis and other chronic inflammatory diseases. Consequently, our proposal is highly significant and broadly impacts a critical area of human health concern.